Likely fearful of attack ads, seven Senate Democrats joined Republicans on Wednesday to block the nomination of Debo P. Adegbile to be the next assistant attorney general for the Civil Rights Division of the Department of Justice.

Three of those Democrats — Chris Coons of Delaware, Mark Pryor of Arkansas and John Walsh of Montana — are on the ballot this fall. The others either had regional ties to the controversy dogging Adegbile or are from red states. Sens. Bob Casey, D-Pa. voted against cloture, as did Democrats Heidi Heitkamp of North Dakota, Joe Donnelly of Indiana and Joe Manchin III of West Virginia.

Adegbile's nomination was the most controversial to hit the Senate floor since Majority Leader Harry Reid, D-Nev., led his party to change the chamber's filibuster rules, requiring only a simple majority for many administration nominees. Wednesday's vote demonstrated the political risk in such a maneuver, squarely putting the burden of confirmation on the majority and politically exposing the most vulnerable members of that majority.

Adegbile was the head of the NAACP Legal Defense Fund in 2011, when the group provided legal representation to Mumia Abu-Jamal. His death sentence for killing a Philadelphia police officer in 1982 was reduced to life without parole.

Opponents of his nomination said they did not believe it was right to confirm an administration official who had defended a client who had murdered a cop. Though much of the uproar over Adegbile's nomination had been regional , Republicans made the pick into a national issue. Senate Minority Leader Mitch McConnell, R-Ky., used his opening floor remarks Wednesday to attack the administration's selection of Adegbile, capping a backlash that was largely pushed by Pennsylvania Republican Patrick J. Toomey.

With the GOP painting Adegbile as an advocate of an "extremist cop killer," Democrats were put in a political box, stuck between potential negative ads and supporting a nominee many believed was qualified for the post. Coons said in a statement the vote was "one of the most difficult" he has cast.